Lightning is a major source of wind turbine component failures. The conventional art of lightning protection is well documented by the international standard IEC 61400-24. In general, the lightning protection system addressed by this standard uses blade conductors connected to the blade rotor hub, which is connected to the main shaft. In turn, the main shaft is connected via slip rings or spark gap to a bed plate, which is connected to the tower via slip rings or spark gap. This system works reasonably well for generators having a gearbox, since an insulating coupling can be used between the gearbox and generator to inhibit lightning from reaching the generator, which is expensive and easily damaged. The drivetrain bearings are difficult to protect adequately, however, since the conductive roller bearings are in contact with the main shaft, which is also conductive. Also, the gears within the gearbox are difficult to protect adequately for the same reason. Pitting of bearing and gear surfaces is the likely result of lightning strikes to this type of protection system.
Such a protection system is generally impractical for a direct-drive wind turbine, however, since it is difficult to isolate the blade rotor from the generator rotor because they are directly connected to each other and are virtually universally made of conductive material, i.e., metal, due to the large loads present. Consequently, the generator of a direct-drive wind turbine is susceptible to damage from lightning strikes to the blades and hub of the blade rotor. While lightning protection systems have been designed to conduct lightning striking the blades away from the generator of a direct-drive wind turbine, lightning strikes to the blade rotor hub remain a large threat to the generator.